1. Field of the Invention
Exemplary aspects of the present invention generally relate to an optical scanner and an image forming apparatus including the optical scanner.
2. Description of the Background Art
Optical scanners employed in image forming apparatuses comprise multiple light sources and a deflector such as a polygon scanner. In order to achieve both high productivity and high imaging quality, typically both the number of light sources and the number of rotations of the deflector are increased.
Increasing the number of light sources increases the cost of the optical scanner as a whole, however, and also requires the deflector to be operated at high speed. A further drawback to this approach is that the amount of heat generated from operation of the deflector increases as the number of rotations of the polygon scanner increases. If the polygon scanner is not isolated from the rest of the optical scanner by a partition member such as a wall, hot air generated and spread by deflection mirrors undesirably and unevenly heats optical elements in the optical scanner, causing optical properties of the optical scanner to deteriorate and thus degrading imaging quality.
Referring now to FIG. 10, there is provided a cross-sectional view of a related-art optical scanner. The related-art optical scanner includes an optical casing 801, a polygon scanner 802, soundproof glasses 803a and 803b, fθ lenses 804a, 804b, 804c, and 804d, a long lens 805a, 805b, 805c, and 805d, first mirrors 806a, 806b, 806c, and 806d, second mirrors 807a, 807b, 807c, and 807d, third mirrors 808a, 808b, 808c, and 808d, dustproof glasses 809a, 809b, 809c, and 809d, an upper cover 810, a bottom cover 801, a lower cover 811, photoreceptor drums 812a, 812b, 812c, and 812d, and optical paths 813a, 813b, 813c, and 813d. 
As illustrated in FIG. 10, even if airspace in which deflection mirrors rotate is in the optical scanner which is isolated from outside to some extent, as the deflector rotates at high speed, tiny particles of foreign substance such as dust unevenly adhere to the deflection mirrors after extended use, adversely affecting reflection characteristics of the deflector. As a result, unity of scan light of the optical scanner is degraded and therefore imaging quality is also degraded.
To address the above-described problems, one related-art optical scanner of an image forming apparatus includes a cover provided substantially above the deflector to seal the deflector within an optical casing of the optical scanner. With this configuration, it is possible to reduce, if not prevent entirely, hot air from scattering and contamination of the deflection mirrors.
One example of such an optical scanner is illustrated in FIG. 11. FIG. 11 is a cross-sectional view of the related-art optical scanner.
The optical scanner in FIG. 11 includes an optical casing 901, a polygon scanner 902, soundproof glasses 903a and 903b, fθ lenses 904a, 904b, 904c, and 904d, first mirrors 906a, 906b, 906c, and 906d, second mirrors 907a, 907b, 907c, and 907d, dustproof glasses 909a, 909b, 909c, and 909d, an upper cover 910, a lower cover 911, photoreceptor drums 912a, 912b, 912c, and 912d, and optical paths 913a, 913b, 913c, and 913d. 
As describe above, in order to achieve high productivity and high imaging quality, the deflector is rotated at high speed. When the deflector is sealed in the optical casing as illustrated in FIG. 11 and rotated at high speed, heat does not get dissipated, thereby increasing the temperature of the deflector. As a result, the temperature of the deflector exceeds its maximum operating temperature and/or the heat migrates to other optical elements through the optical casing, thereby degrading the optical characteristics.
In view of the above, various approaches have been proposed in an attempt to solve the problem. For example, one related-art optical scanner includes partitions provided to a place where a rotary deflector is mounted in a casing of an optical scanner so as to reduce adverse effect of heat generated by the rotary deflector in the optical scanner and to protect the deflector from dust.
In another related-art optical scanner, a shielding member is provided between the deflector and the optical elements so as to block airflow and thus reduce adverse effect of heat generated in the optical scanner.
Still another related-art optical scanner includes a cover for sealing a mounting portion of the deflector from the outside air. The cover includes ribs that project toward the deflector to prevent heat from scattering, thereby cooling inside the optical scanner effectively.
Although advantageous, such optical scanners still suffer from a drawback in that, when the deflector is rotated at high speed while sealed within the optical casing of the optical scanner, the temperature of the deflector increases excessively and heat migrates to the optical elements, causing degradation of optical characteristics of the optical scanner.